Fast breathing for a premature baby

I have a premature baby born at 36 weeks when he came home he caught rsv. But since the hospital I noticed he breaths really fast. And when he eats it seems worst. Now that he feels better Im worried because its still fast his doctor sent me to another doctor to listen to it and they both said its fast then itll slow down but to keep an eyeon it. What could cause this and should I take it more serious as to maybe get someone to look at his heart . He is 6 weeks old.